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ABC News

NSW dam levels reach 100 per cent capacity in many regions

A third consecutive La Nina settling in to the east coast of Australia will almost certainly bring more rain to dams already filled to the brim.

More than half of regional NSW's state-owned dams are at or above capacity, and authorities say they they have not been this full since the 1990s.

100 per cent not what it seems

Reaching capacity is not necessarily cause for panic for flood-prone communities. 

State Emergency Service (SES) community capability officer Jake Hoppe said dam levels were measured a little differently, and 100 per cent full did not mean overtopping.

"100 per cent just means that's the optimal level and anything above that is extra water that possibly could be released," he said.

"Dams will strategically release water prior to a rain event if it's anticipated."

The fine line of water releases

But with heavy rainfall once again, there are concerns about how such full dams will cope.

Burrendong Dam in the state's west is at 130 per cent capacity.

Water releases mean towns such as Warren and surrounding farms are in prolonged major flood.

Water NSW spokesperson Tony Weber said releasing water from full dams into saturated catchments was a balancing act.

"In normal circumstances you can release that water into the river once the tributaries start to recede but what we have right now is very high tributary flows," he said.

"Even just 30 to 40 [millimetres of rain] is driving huge amounts of water into the dam that then needs to at some point be released in the river downstream."

Mr Weber said there had not been enough of a break in the rain to be able to safely release water.

"We don't want to add dam water onto water that is already running from downstream tributaries," he said.

"But those windows of opportunity to make preparations for the next rain event are fleeting, if at all."

Near Tamworth, a decision had been made to put off larger releases to allow tributaries to peak downstream and prevent impacts on floodwaters at Gunnedah.

They were later increased to 15,000 megalitres a day about midday on Friday.

Should we be worried?

Water NSW had to drastically increase releases from Burrinjuck Dam near Yass to 100,000 megalitres a day last month after a quarter of its capacity unexpectedly flowed in overnight.

It caused major flooding at Gundagai but Water NSW said at the time that if it had not being for the dam, floodwaters would have rushed through the system faster, causing more devastation downstream.

The SES says many dams are equipped to cope with more rain, with regular releases designed to allow enough airspace for a major downfall.

"There's hundreds of dams in NSW and it's something that most people won't notice," Mr Hoppe said.

"Water NSW provide some really up to date and timely information; we just ask people research which dams may impact them."

Why not drop dam levels right down?

The primary purpose of dams is to make sure that they capture water in wet periods so that it is available in times of drought.

Mr Weber said that responsibility was front of mind for authorities when releasing water given the "extremes of drought".

"Unless there is a certainty that you are going to receive huge volumes of water, the notion of dropping dams or holding them 20 per cent lower than what they might ordinarily be for convenience's sake really risks disaster," he said.

Mr Weber said a long-term outlook on dam management was crucial to ensuring water security. 

"We are not just managing them for the days and weeks ahead but we really need to think about water security for years ahead." 

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